In order to print an image with an image forming apparatus such as a copy machine, printer or facsimile, first, a photosensitive material is exposed according to image information obtained from an original document or a predetermined image source, thereby forming a latent image. Then, a developer is utilized to apply toner to the latent image, in order to develop the image. The developed image is then transferred to and fixed on a sheet of paper.
To form a color image, respective developers for yellow, magenta, and cyan developing are generally used. In addition, a fourth developer for black developing is typically also utilized. Being contained in respective developing devices, different color developers are sequentially selected and the different color toners applied to portions of latent image of the photosensitive material and then transferred to a sheet of paper. Color image forming apparatus with a plurality of developers respectively containing the different color toners are divided into a fixed type image forming apparatus and a rotary type image forming apparatus according to the arrangement of the developing devices.
In the fixed type apparatus, a plurality of developing devices are radially disposed around a cylindrical photosensitive drum so that a selected developing device applies toner to the photosensitive drum at the fixed position of the developing device. An example of this type of apparatus is described in U.S. Pat. No. 4,063,724.
In the rotary type apparatus, the developing devices are radially incorporated in a rotator installed adjacent to the photosensitive drum. The rotator rotates so that the selected developing device moves to the development area of the photoconductive drum. An example of this type of apparatus is described in U.S. Pat. No. 4,615,612.
The fixed type apparatus may not be easily made into a compact apparatus because a large-diameter photosensitive drum is needed to provide sufficient space for positioning the plurality of developing devices. In addition, since the distances from the latent image on the photosensitive drum to the respective developing devices are different, the residual potential of the latent image is different at respective development devices. For this reason, the density of the respective colors developed is not uniform, thereby resulting in poor picture quality.
In the rotary type color image forming apparatus, at least one developing unit turns upside down when the rotator rotates, so the toner leaks and contaminates the interior of the apparatus. This is detrimental to the maintenance of the apparatus. Additionally, a high driving torque is required to drive the rotator. Therefore, when the rotator rotates, severe vibration and loud noise are generated, which prevent stable operation of the apparatus. This vibration results in poor picture quality.
U.S. Pat. No. 5,014,095 discloses another image forming apparatus that is different from the fixed and rotary types. This image forming apparatus comprises an elevation box in which four developing units are sequentially stacked. The elevation box is lowered or raised in the same manner as that of an elevator so that a developing unit selected on one side of a photosensitive drum moves to the development position of the photosensitive drum. In this type of apparatus in which the developing devices are lowered or raised, the problem of non-uniformity in image density due to different potentials of the latent image relative to respective developers is essentially eliminated and contamination due to the leakage of toner is prevented. However, making this type of apparatus more compact is difficult because sufficient space for moving the elevation box accommodating the developing devices is required. Further, a high driving torque is necessary, which is a hinderance to the stable operation of the apparatus for the reasons described above. Accordingly, enhancement of picture quality is not effective.
To solve the above described problems, an image forming apparatus comprising a new driving method disclosed in Korean patent application No. 94-1375 is utilized. This image forming apparatus comprises a structure providing for synchronously moving a relatively small and light transfer unit and an exposure unit, rather than fixing heavy and voluminous developers, thereby enabling the construction of a very compact apparatus with stable operation.
An image forming apparatus that is more stably driven by fixedly mounting an optical device of the exposure unit is disclosed in Korean patent applications Nos. 94-7658 and 94-7659, both of which have been incorporated into U.S. Pat. No. 5,532,816. In the apparatus disclosed in these patent applications, one rotating mirror and a plurality of fixed mirrors are used to reflect light projected from the exposing optical device onto respective photosensitive drums of the plurality of the image forming units.
Given that the optical path lengths from the exposing optical device to the respective photosensitive drums are different in the above exposure systems, the magnitudes of the latent images formed in the respective photosensitive drums are slightly different. As a result, an offset is generated in a color image transferred to a sheet of paper. A focusing operation required to solve the offset problem causes the optical device to be undesirably complicated.